9

LTC1590

Op Amp Selection

To maintain the excellent accuracy and stability of the

LTC1590 thought should be given to op amp selection.

Fortunately, the sensitivity of INL and DNL to op amp offset

has been significantly reduced compared to competing

offset. In addition, because the DAC’s equivalent output

fixed reference applications this causes gain, INL and DNL

error. For multiplying applications, a code-dependent, DC

output voltage error is seen. At zero scale the DAC output

error is equal to the op amp offset, and at full scale the

output error is equal to twice the op amp offset. For

example, a 1mV op amp offset will cause a 0.41LSB zero-

scale error and a 0.82LSB full-scale error with a 10V full-

scale range. The offset caused INL error is approximately

10V full-scale range, the INL degradation will be 0.17LSB

and DNL degradation will be 0.03LSB.

Op amp bias current causes only an offset error equal to

amp bias current causes a 1.1mV DAC offset, or 0.45LSB

for a 10V full-scale range. It is important to note that

connecting the op amp noninverting input to ground

through a resistor will not cancel bias current errors and

should never be done! Similarly an offset caused by op

amp bias current should not be adjusted by using the op

amp null pins since this increases offset between DAC

OUT1 and OUT2 pins, causing INL, DNL and gain errors.

If op amp offset error adjustment is required, the op amp

input offset voltage (the voltage difference between OUT1

and OUT2) should be nulled.

Grounding

As with any high precision data converter, clean ground-

ing is important. A low impedance analog ground plane

and star grounding should be used. OUT2 carries the

complementary DAC output current and should be tied to

the star ground with as low a resistance as possible. Other

ground points that must be tied to the star ground point

13

14

11

12

15

DATA IN

SERIAL CLOCK

CHIP SELECT/DAC LOAD

DATA OUT

CLEAR

16

98

1

2

10

1590 TA07

3

5

7

1

8

6

4

5

6

2

3

4

5V

±10V

±10V

7

33pF

0.1

µF

0.01

µF

33pF

15V

–15V

0.01

µF

D

4096

()

DAC B

DAC A

24-BIT

SHIFT

REG

AND

LATCH

CLK

CS/LD

CLR

DGND

AGND

LTC1590

OUT1 B

OUT2 B

OUT2 A

OUT1 A

+

–

+

–

1/2

LT1358

1/2

LT1358

Dual Programmable Attenuator